1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) using a magnetoresistive effect, magnetic random access memory manufacturing method, and magnetic random access memory write method.
2. Description of the Related Art
Magnetic random access memories (MRAMs) using a tunneling magnetoresistive (TMR) effect have been proposed recently as information storage elements (e.g., Roy Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC 2000 Technical Digest, p. 128).
A memory cell of a magnetic random access memory comprises an upper write wiring, lower write wiring, and magnetic tunnel junction (MTJ) element provided between the upper and lower write wirings. The MTJ element has a fixed layer, recording layer, and tunnel barrier layer sandwiched between the fixed layer and the recording layer.
In a so-called toggle cell, the direction of axis of easy magnetization of the MTJ element tilts 45° with respect to the running direction of the upper or lower write wiring. The recording layer of the MTJ element has a three-layer structure of ferromagnetic layer/nonmagnetic layer/ferromagnetic layer. The two ferromagnetic layers are anti-ferromagnetically coupled through the nonmagnetic layer.
In a write operation of such a toggle cell, first, current is sequentially supplied to the upper and lower write wirings. Then, supply of the currents is stopped in the same order as that of its start. With this process, magnetization reversal occurs regardless of the initial magnetization state. Hence, before the write, data of the MTJ element is read out to determine the current cell state. After that, the write operation is started as needed.
The resistance state of the MTJ element changes between “1” and “0” depending on whether the magnetization direction of the fixed layer and that of the ferromagnetic layer of the recording layer located on the fixed layer side is parallel or anti-parallel. The read operation can be executed by extracting the resistance state.
The toggle cell is hardly affected by a variation in magnetization reversal threshold value of each MTJ element in principle and can advantageously be used for increasing the scale of magnetic random access memories. However, it is not yet the standard technique of magnetic random access memories because of problems such as the large write current and low write speed.